1. Field of the Invention
This invention relates to the construction of circuit breakers which can be used for DC circuit breaking, and in particular to hybrid circuit breakers consisting of a vacuum circuit breaker portion and a gas circuit breaker portion connected in series.
2. Description of the Prior Art
Usually DC circuit breakers comprise a condenser/reactor circuit in parallel with a circuit breaking portion, and circuit breaking is performed by discharging the charge stored in the condenser through a reactance. The oscillating current which is then produced is superimposed on the DC current of the main circuit to artificially create a current zero point. The contacts of the circuit breaking portion are opened in synchronization with this current zero point, to achieve circuit breaking. However, if a vacuum circuit breaker is used as the circuit breaking portion, sufficient safety cannot be at present achieved by the use of such a vacuum circuit breaker alone, since the characteristics of vacuum circuit breakers make them liable to restriking of the arc. So-called hybrid circuit breakers, as described in U.S. Pat. No. 4,159,498, have therefore been devised, in which circuit breaking is achieved by a series combination of a vacuum circuit breaker and gas circuit breaker, the former performing the initial circuit breaking while the latter is allotted the task of recovery of insulation after circuit breaking.
The excellence of such hybrid circuit breakers for DC circuit breaking has been proved. Such hybrid circuit breakers comprise a gas circuit breaker portion and vacuum circuit breaker portion connected in series in a closed tank, which is filled with insulating gas (for example SF6 gas). However, when a vacuum circuit breaker portion and gas circuit breaker portion are accommodated together in the same tank, there is the following drawback. Specifically, in the case of an SF6 gas circuit breaker, when the current is broken, decomposition gas is produced. Most of this recombines to form SF6 again, but some remains as decomposition gas, and may cause corrosion of the material of the circuit breaker. For this reason the circuit breaking portion of an SF6 gas circuit breaker is made of decomposition gas-proof material. However, the vacuum valve container which constitutes the vacuum circuit breaking portion is made of borosilicate glass, which is liable to corrosion by decomposition gas. When a gas circuit breaking portion and vacuum circuit breaking portion are accommodated together in the same tank, some means of preventing corrosion must therefore be adopted.